1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus using a two-component developer containing a magnetic carrier and a nonmagnetic toner.
2. Description of the Related Art
A two-component developing method using a toner and a carrier and a one-component developing method using no carrier are known as developing methods in image forming apparatuses. The two-component developing method has advantages of having a good chargeability of the toner by the carrier and a longer operating life, whereas it has disadvantages of making a developing device large and complicated and varying image quality depending on the durability of the carrier. Further, the one-component developing method has advantages of making the developing device compact and having a good dot reproducibility, whereas it has disadvantages of generally making a developing roller and a supply roller less durable and making a consumable cost more expensive due to the exchange of developing devices on a regular basis. Further, the supply of the toner having such a charging property as to be developed on the developing roller is not suitable for high-speed processing apparatuses, which has presented a problem to the speed-up of the image formation.
There has been known a so-called touch-down developing method taking advantages of characteristics of the above both developing methods. The touch-down developing method uses a two-component developer containing a toner and a carrier, forms a toner layer on a developing roller with a magnetic brush having the sufficiently charged toner, and develops an electrostatic latent image formed on a photoconductive member in a non-contact manner with the toner held on the developing roller.
The touch-down developing method is a developing method capable of high-speed image formation and is applicable to a developing device of a one-drum color superimposing type in which a plurality of color images are successively formed on a photoconductive member; of a tandem type in which a plurality of electrophotographic processing members are arranged side by side and color images are formed and superimposed on a transfer material (sheet) in synchronism with the conveyance of the transfer material; of a tandem type in which a plurality of electrophotographic processing members are arranged side by side along an intermediate transfer member (transfer belt) and color images are superimposed on the intermediate transfer member; and of other types.
In the case of tandem image forming apparatuses, a plurality of electrophotographic processing members is arranged side by side. Thus, if developing rollers and magnetic rollers are transversely arranged with respect to photoconductive members, the electrophotographic processing members themselves have a large width, which hinders the miniaturization. Therefore, miniaturized image forming apparatuses have been proposed in which the developing rollers and the magnetic rollers as the electrophotographic processing members are arranged above or below the photoconductive members to make the developing devices vertically long.
As a prior art on such a technology, U.S. Pat. No. 3,929,098 (lines 10 to 43, second column) discloses a developing device in which a developer is caused to head for a donor roller (developing roller) using a magnetic roller to transfer a toner onto the donor roller, thereby forming a thin toner layer. However, according to this method, a toner charge control is complicated and it is necessary to apply a high surface potential and a large developing electric field to a photoconductive member. Further, it is difficult to refresh the toner on the donor roller, which was not used for development, and a toner adhering state and a potential difference of the toner on the donor roller vary if a toner consumed region and a toner non-consumed region are present on the donor roller. Such variations are likely to cause a phenomenon in which a part of a previously developed image appears as a ghost image during the next development, i.e. a so-called history phenomenon.
In order to solve this problem, Japanese Unexamined Patent Publications Nos. 2003-21961 and 2003-21966 disclose developing devices each comprising a magnetic roller in which a magnetic member for holding the magnetic brush formed of a two-component developer containing a carrier and a toner is fixed; a developing roller for forming a thin toner layer by the abrasive contact with the magnetic brush held by the magnetic roller; and a power supply for forming an alternating-current bias between the developing roller and a photoconductive member. In each of these developing devices, a latent image on the photoconductive member is developed with the toner caused to fly from the thin toner layer formed on the developing roller by the alternating-current bias, thereby preventing an occurrence of ghost at the time of development while avoiding an occurrence of an image fog. However, according to this method, a highly accurate control is required to balance the alternating-current bias formed between the developing roller and the photoconductive member and direct-current biases applied to the developing roller and the magnetic roller.
Further, Japanese Unexamined Patent Publication No. 2003-280357 discloses a developing device comprising a magnetic roller and a developing roller similar to the above and adapted to apply an alternating-current bias superimposed with a direct-current bias to the developing roller. Here, by setting a duty ratio of the alternating-current bias to 10 to 50%, the toner attraction (collection) from the developing roller to the magnetic roller is increased to solve the contamination of the developing roller with the toner. However, in the developing device of this type as well, a highly accurate control is required to balance the alternating-current bias applied to the developing roller and direct-current biases applied to the developing roller and the magnetic roller. Therefore, there has been a demand for technology requiring less control accuracy.
Japanese Unexamined Patent Publication No. 2001-134050 discloses a developing device using a one-component developer, comprising a developing roller held in contact with a photoconductive member and a supply roller held in contact with the developing roller, and constructed such that a toner is supplied to the developing roller by the supply roller and develops a latent image on the photoconductive member while being frictionally charged by a restricting blade on the developing roller to form a thin layer. In this device, an alternating-current voltage is applied to the developing roller, thereby preventing a problem that it is difficult to develop low-density images and thin line images and a problem that density nonuniformity occurs due to an increase in a toner charge amount and making it easier to scrape off (collect) the toner not having been used for development. However, an image fog occurs if the alternating-current voltage applied to the developing roller for forming a developing electric field is increased, whereas the effect of scraping off the toner not having been used for development is reduced if the alternating-current voltage is decreased. It is disclosed to apply an alternating-current voltage also to the supply roller and let the two alternating-current voltages have the same frequency, but different phases in order to solve this problem. However, the developing device is of the type using the one-component developer and constructed such that the photoconductive member and the supply roller are in contact with the developing roller and, if the developing devices of such a type in which the photoconductive member and the developing roller are in contact are used in a tandem image forming apparatus, a torque variation of a transfer belt might be caused to promote a color drift as a weak point of the tandem image forming apparatus.
In view of the above, Japanese Unexamined Patent Publication No. 2005-242281 discloses a developing device including a magnetic roller in which a magnetic pole member holding a magnetic brush is fixed, a developing roller to be rubbed by the magnetic brush held in the magnetic roller for the formation of a thin toner layer, a power supply for applying an alternating-current bias to the developing roller and another power supply for applying an alternating-current bias, which is a rectangular wave having the same frequency as, an opposite phase to and an inverted duty ratio of the above alternating-current bias, to the magnetic roller. This device makes it easier to form the thin toner layer on the developing roller and to collect the toner from the developing roller by increasing a potential difference between the alternating-current bias of the developing roller and that of the magnetic roller. This developing device balances the respective biases formed between the developing roller and a photoconductive member and between the developing roller and the magnetic roller so that image developability can be maintained without changing the potential difference between the photoconductive member and the developing roller at all even if it should be used in a tandem image forming apparatus.
However, in order to cope with faster printing, miniaturization and even higher image quality in image forming apparatuses of recent years, it is asked for to rotate the photoconductive member at a higher speed, to make the diameter of the photoconductive member smaller and to make toner particles smaller. If time required to pass a developing area is shortened due to the smaller diameter and faster rotation of the photoconductive member and the smaller diameter of the developing roller, it is necessary to increase a developing electric field or reduce toner adherence to the developing roller in order to improve the developability on the photoconductive member. Further, if time required to pass a toner layer forming area is shortened due to the smaller diameter and faster rotation of the developing roller and the smaller diameter of the magnetic roller, it is necessary to reduce the toner adherence to the developing roller while intensifying the electric field for collecting the toner from the developing roller. If the toner particles are made smaller, it is necessary to generate a strong electric field between the photoconductive member and the developing roller to increase a force for causing the toner to fly from the developing roller to the photoconductive member while suppressing an increase of the toner adherence to the developing roller surface and also to intensify the electric filed between the developing roller and the magnetic roller for collecting the toner from the developing roller to the magnetic roller.
However, since the biases applied to the developing roller and the magnetic roller become a composite bias between the developing roller and the magnetic roller, the bias applicable to suppress a discharge while maintaining the developability and collectability is restricted in its phase, cycle and waveform, which has hindered the miniaturization and the higher speed. Specifically, the toner on the developing roller comes into contact with the magnetic brush many times according to the rotation of the developing roller even after being supplied to the developing roller by the magnetic brush, and is exposed to the electric field applied between the magnetic brush and the developing roller each time. Thus, if the electric field acting in a direction to supply the toner toward the developing roller is increased for the higher speed or the like, the toner is likely to firmly adhere to the developing roller. This, for example, hinders the toner supply from the developing roller to the photoconductive member and makes it difficult to collect the toner from the developing roller to the magnetic roller. As a result, a range in which the bias formed between the developing roller and the photoconductive member and the one formed between the developing roller and the magnetic roller are balanced becomes even narrower.
In order to improve the developability by adjusting the toner adherence and volume resistance of the developing roller surface, U.S. Pat. No. 6,674,986 discloses technology for covering a developing roller surface with an insulating member. Further, Japanese Unexamined Patent Publication No. H11-249414 discloses technology for covering a developing roller surface with a urethane resin or the like. However, with the technologies of these documents, if a toner particle diameter is made smaller, the highly charged fine toner has a strong adhering force to the developing roller. Thus, unless an electric field generated between the developing roller and a photoconductive member is stronger than the toner adhering force to the developing roller, it has been difficult to develop an image. Further, unless the surface of the resin coating formed on the developing roller has a certain surface roughness, there has been a likelihood of affecting the transfer of the toner to the magnetic roller for toner collection.
With the technologies of the patent literatures described above, it has been difficult to improve the developability on the photoconductive member while coping with the formation of the thin toner layer on the developing roller and the collection of the toner from the developing roller by balancing the bias formed between the developing roller and the photoconductive member and the one formed between the developing roller and the magnetic roller while maintaining good toner adherence to the developing roller in the development process asking for the faster rotation and the smaller diameter of the photoconductive member and the smaller toner particles.